Electrolytic production of zinc



Patented July 16, 1963 3,098,019 ELECTROLYTIC PRODUCTION OF ZINC RichardF. Pagel, Believille, IlL, assignor to American Zinc, Lead and smeltingCompany, St. Louis, Mo., a corporation of Maine No Drawing. Filed Mar.9, 1962, Ser. No. 178,568 4 Claims. (Cl. 204-55) The invention relatesgenerally to the electrolytic production of zinc, and particularly to amethod of lowering the lead content of zinc produced by the electrolyticprocess.

This is a continuation-in-part of my copending application Serial Number87,083, filed February 6, 1961, now abandoned.

In the process commonly used for the production of electrolytic zinc, anelectric current is passed between an electrode of metallic lead or leadalloy, which is made anodic, and an electrode of aluminum made cathodic,in an acidified zinc-bearing electrolyte, usually a sulfate. Metalliczinc is deposited upon the cathode in a relatively pure form, usually atleast 99.9% pure. Of the impurities present, lead is the mosttroublesome, although normally present in amounts of only a fewthousandths of one percent. Even at this level it is considered highlyobjectionable to the making of zinc base die casting alloys.

The problem of reducing the lead content of electrolytic zinc has beenrecognized for a long time. Numerous previous inventors have used ordevised various means in attempting to decrease the lead content inelectrolytic zinc. Both organic, as well as inorganic, additives havebeen added to the electrolytic solution and both have proved successfulin the reduction of the lead content. Similarly, organic materials areWell known as additives for purposes other than for the suppression oflead. However, only by the use of relatively large quantities of theseadditives can a commercially acceptable process be obtained, whether itbe for the suppression of lead or for improving the covering power ofthe deposited zinc.

Flett, Patent No. 2,195,409, suggests the use of an alkyl aromaticsulfonic acid compound for use in electro-deposition of zinc to improvethe covering power of the deposited metal and eliminate pitting andpinholing. Quantities of 0.1 to 0.3 gram of the additive per liter ofelectrolyte were used in this process. However, no mention is made ofthe fact that by the addition of this material the lead content of thedeposited zinc is decreased.

Wernlund, Patent No. 2,600,352, teaches the use of certain organicketones of the aliphatic type, such as methyl ethyl ketone and acetone,as additives to an electroplating solution to be used as brightencrs forthe zinc electroplated. He states that a concentration of at least about3 grams per liter of electrolyte is required to obtain a commerciallyacceptable plate. Here again, no mention is made of the fact that theaddition of these organic ketones will lower the lead content ofdeposited zinc.

Nachtman, Patent No. 2,370,986, suggests the use of nicotine, thiourea,ammonium thiocyanate and the sodium salts of the sulfate ester of amixture of long chain aliphatic alcohols as additives in a zincelectrolytic bath. However, Nachtman makes no mention of the effectwhich this additive has on the lead content in the deposited zinc. Largequantities of these additives are used in this process to 42.5 grams ofthe additive per liter of electrolyte.

Gray, in Patent No. 2,407,489, suggests the use of anthraquinonesulfates to retard the pitting and pin-holing effect of electrolyticallydeposited zinc. Gray uses quantities of 0.5 to 5 grams of additive perliter of electrolyte, but makes no mention of the fact that theanthraquinone decreases the lead content of the deposited zine.

Lowe, in Patent No. 2,471,965, attempts to reduce the lead content ofelectrolytic zinc by the addition of 2 to 10 pounds of cresylic acid andup to 6% pounds of pine oil per ton of zinc deposited. Cresylic acid isa mixture of phenol and its homologues. And he suggests that by theaddition of these additives in the aforestated amounts the lead contentof the deposit of zinc will be below the maximum requirement of zincpurity established by the British Ministry of Supply, which was, at thattime, 0.003% lead.

Certain inorganic salts have been suggested as additives to reduce theamount of lead appearing in deposited zinc. Yeck and Lebedelf, in PatentNo. 2,539,681, suggest the addition of barium or strontium salts.Strontium is added as strontium carbonate at a rate between 2.2 and 45pounds of strontium carbonate per ton of cathode zinc produced. By thisaddition of 45 pounds strontium can bonate to the electrolyte, the leadcontent was reduced to about 0.00064%, average.

One of the objects of the present invention is to provide a method ofproducing electrolytic zinc with a low lead content, easily andconsistently.

Another object is to provide a commercially acceptable electrolyticprocess for the production of zinc of low lead content by the use ofminute quantities of an organic additive to the electrolyte.

Other objects will become apparent to those skilled in the art in thelight of the following description.

In accordance with this invention, generally stated, a process ofelectrowinning zinc is provided, which includes the addition to theusual zinc electrolyte of a halogenated benzoquinone, of whichchloranilic acid (2,5-dichloro- 3,6-dihydroxyquinone) is the preferredexample. The electrolyte in this process contains an amount of additiveso small as compared with additives known heretofore, as to be of awholly different order of magnitude.

In the case of halogenated benzoquinones, the preferred operable limitsare from about 0.0001 to 0.1 gram per liter of electrolyte. The use ofmuch more of the additives than about 0.1 gram per liter of electrolyte,results in the reduction in current efficiency and blackening of thesurface of the zinc deposit.

The following example is illustrative of the process of the presentinvention:

Example 1 An electrolytic solution taken from a commercialelectrowinning plant, having the following metallic content,

G./'liter Zn Mn 2.50 Pb 0.01 Cd 0.001 Fe 0.035 As 0000i was put into atest plant circuit. Electrolysis of this solution was started and forthe first six days during which period no lead suppression reagents wereadded, the lead Beginning with the seventh day, an amount of chloranilicacid amounting to approximately 0.007 pound per ton of cathode zinc(0.0004 gm. per liter of electrolyte was added to the electrolyte in thecircuit, at which time there was an immediate reduction in the amount oflead in the cathode zinc deposited thereafter, as shown by the followingassays:

Day: Lead assay, percent 7 0.0019 8 0.0018 9 0.0011 10 0.0015 11 0.001512 0.0013 13 0.0013 14 0.0013 15 0.0017 16 0.0014 17 0.0014

1 shorted electrode in one cell.

The electrotyte in the test circuit was maintained at a temperature ofapproximately 43 C.

It has been found that the effectiveness of chloranilic acid as a leadsuppressant increases as the temperature of the cells decreases. Thus,cathode zinc assays of approximately 0.0005% lead have been obtained attemperatures of about 32 C.

In normal zinc plant operations the tendency is for the lead assay todrop to 10.004 during the first four or five days after electrolysis isstarted with clean anodes. After this time, at temperatures of about 40C. there is little or no decrease in the lead assay unless a suppressingagent is added.

Although the electrolysis in this case was carried out using the leadsilver alloy anode that has become the industry standard, the sameprinciple of operation will prevail if a chemical or ordinary lead anodeis used.

We have also found that, in both commercial plant and test plant scaleoperations, greater effectiveness of chloranilic acid as a suppressantis obtained if the man ganese content is held below 2.5 grams per liter.However, even if the manganese content is allowed to be more than thisamount, the chloranilic acid would still be active as a leadsuppressant, but its efiectivencss will be slightly deterred.

Among the other substituted-p-benzoquinones which have been found usefulin the present invention are 2,6- dichloro-p-benzoquinone, chloranil(tetrachloro-pdvenzoquinone) and trichloro-p-benzoquinones. It isbelieved that other halogenated p-benzoquinones, as well as other di,tri, and tetra substituted p-benzoquinones, may also be applicable inthe present invention.

While only a single illustrative embodiment of the process of thepresent invention has been set forth, it is to be understood that theinvention is not limited to the particular material mentioned, or to theparticular proportions set out.

Many changes in and modifications in the method set forth may be madewithout departing from the spirit of the invention or sacrificing any ofits advantages.

Having thus described the invention, what is claimed and desired to besecured by Letters Patent is:

1. In the method of electrolytically depositing high quality zinc, thestep comprising adding to an acidic zinc electrolyte a quantity of ahalogenated p-benzoquinone within the range of 0.0001 and 0.1 gram perliter of said electrolyte.

2. The method of claim 1 wherein the halogenated pbenzoquinone ischloranilic acid.

3. The method of claim 1 wherein the lead content of the deposited zincis less than 0.002%.

4. The method of claim 1 wherein the halogenated benzoquinone is presentin an amount within the range of 0.000 1 to 0.001 gram per liter ofelectrolyte.

References Cited in the file of this patent UNITED STATES PATENTS GraySept. 10, 1946 Wernlund June 10, 1952 OTHER REFERENCES

1. IN THE METHOD OF ELECTROLYTICALLY DEPOSITING HIGH QUALITY ZINC, THESTEP COMPRISING ADDING TO AN ACIDIC ZINC ELECTROLYTE A QUANTITY OF AHALOGENATED P-BENZOQUINONE WITHIN THE RANGE OF 0.0001 AND 0.1 GRAM PERLITER OF SAID ELECTROLYTE.